The problem can be corrected by upgrading the affected package to
version 1.8.4-36ubuntu0.1. In general, a standard system upgrade is
sufficient to effect the necessary changes.

Details follow:

Several buffer overflows have been discovered in libgd's PNG handling
functions.

If an attacker tricked a user into loading a malicious PNG image, they
could leverage this into executing arbitrary code in the context of
the user opening image. Most importantly, this library is commonly
used in PHP. One possible target would be a PHP driven photo website
that lets users upload images. Therefore this vulnerability might lead
to privilege escalation to a web server's privileges.